1. Field of the Invention
The invention relates to data encoding and more particularly to a method and device for reading a high density self-clocking bar code.
2. Background of the Invention
The recent development of low cost LSI circuit technology lead to a boost in microprocessor applications. Data can now be processed at a competitive cost. But prior to any processing said data should be collected and methods should be available for coding and decoding these data. Several coding methods have already been proposed among which probably the most commonly used is the so called bar coding method. According to this method, the information to be coded is converted into printed rectangular marks and spaces, the respective width of which bear the coded information. For instance, account references are printed on checks using such a method; similarly prices and stock references are printed on tags stuck to the goods carried by supermarkets, etc. . . . Optic or magnetic readers are then used to read this information, convert it into binary coded data, and then feed the latter into a data processing system which balances the customer's account, establishes money transfers and or bills and executes all kind of data processing operations at increased rates and reliability, but lower costs.
Since the code is based on the widths of marks and spaces reading is performed by scanning the printed document either by hand or by machine propelled means. In both cases, accuracy and reliability of decoding rely on the sensitivity of the decoding means used to scanning speed variations, i.e. on their ability to decode properly under varying scanning conditions. In addition, most applications require high density printed bar codes, which adds to the decoding difficulties, since mark and space dimensions become smaller and smaller.
Several types of factors may be acted upon to improve bar coding reliability. Some deal with the type of code itself. In that respect, a significant improvement is achieved with codes such as the one disclosed in U.S. Pat. No. 3,723,710 assigned to the same assignee as this application, as well as the one disclosed in IBM Technical Disclosure Bulletin, Vol. 21, No. 7, December 1978, pages 2883-2884. The latter deals with the so called Every-Transition-A-Bit (ETAB) bar code. Other factors to be acted upon relate to the decoding means which might be made more accurate by improving both the decoding process and the decoding equipment.